This invention generally pertains to the field of liquid crystal displays (LCDs). In particular, the invention relates to high density reflective LCDs and improving their quality and efficiency.
High density reflective LCDs are generally known. FIG. 1 shows a partial cross-section of a generic embodiment of a prior art high density reflective LCD. Generally, the LCD is supported by a silicon substrate 10. Secondary electrodes 12a, 12b, 12c are fabricated on or within substrate 10. Above substrate 10 is liquid crystal 14, which is contained by transparent electrode 16. Above transparent electrode is a second substrate 18, also transparent.
Fabricated below secondary electrode 12a, 12b, 12c in substrate 10 are electronic circuits 20a, 20b, 20c, which interface with secondary electrodes 12a, 12b, 12c. Such electronic circuits 20a, 20b, 20c are also known in the art, and provide a voltage level at the respective secondary electrode 12a, 12b, 12c that alters the state of the liquid crystal 14 adjacent the electrodes 12a, 12b, 12c. (Such secondary electrodes are referred to as xe2x80x9cpixelsxe2x80x9d by those skilled in the art.)
It is the state of the liquid crystal 14 that determines whether and how much light is transmitted. As noted above, the embodiment shown in FIG. 1 is a reflective LCD. Thus, polarized light is projected downward through substrate 18 and transparent electrode 16. If the liquid crystal 14 above the secondary electrode 12a, for example, is in a transmissive state, then the polarization of the light is altered as it passes through the liquid crystal 14 and reflected by secondary electrode 12a. This change in polarization allows the light to transmit through a polarizer positioned externally (not shown in FIG. 1) and, consequentally, the pixel appears bright.
On the other hand, if the liquid crystal 14 above secondary electrode 12a is not in a transmissive state, then the polarization of the light incident on secondary electrode 12a is unaltered and no light passes the external polarizer. Consequently, the pixel corresponding to electrode 12a is dark.
The LCD, of course, is made up of an array of many secondary electrodes (or pixels), such as electrodes 12b and 12c shown in the cross-section of FIG. 1. The states of these electrodes, and the corresponding state of the liquid crystal 14 above each one, will determine the state of the corresponding pixel. Also, as described further below, the state of the liquid crystal 14 may also provide for partial transmission of the light, resulting in a lower intensity glow of the respective pixel.
FIG. 2 is a top view of the array of an LCD such as that shown in partial cross-section in FIG. 1. The pixels of the display corresponding to secondary electrodes 12a, 12b, 12c are marked in FIG. 2. From this perspective, the driving electronics 20a, 20b, 20c would be beneath the electrodes 12a, 12b, 12c and, because of this, the pixels can be positioned closer together on the substrate, resulting in a high xe2x80x9cfillxe2x80x9d factor. (Fill is defined as the area of the secondary electrodes or pixels divided by the area of the supporting substrate. In FIG. 2, this would be equivalent to the square of the width of an electrode (w) divided by the square of the pitch.) A high density reflective LCD can have a fill factor on the order of 0.9 and higher.
Also visible in FIG. 2 are a series of xe2x80x9cspacer beads.xe2x80x9d The spacer posts are not visible in the cross-sectional illustration of FIG. 1, but serve to set the liquid crystal cell gap between secondary electrodes 12a, 12b, 12c and common electrode 16. The spacer beads shown in FIG. 2 are comprised of a series of plastic beads that are randomly positioned between the substrates 10, 18. The spacer posts can be constructed by depositing and patterning an insulating later on the substrate 10. The beads set the liquid crystal cell gap. The beads can be seen when the display is in operation, so reduction of the number of beads needed to set the liquid crystal gap has been pursued.
Referring to FIG. 3, a schematic of the state of the liquid crystal 14 directly above secondary electrode 20a is shown as a function of the voltage of the secondary electrode. (The portion of the liquid crystal 14 shown in FIG. 3 corresponds to the dashed area shown in FIG. 1.)
The alignment of the liquid crystal molecules when there is a low voltage (referred to as xe2x80x9c0 Vxe2x80x9d) is shown in cross-section to be tilted with respect to the axis between the secondary electrode 12a and the transparent electrode 16. (If viewed from above, the molecules would form a helical structure.) In the xe2x80x9crelaxedxe2x80x9d state shown, light is transmitted; thus, the liquid crystal shown in FIG. 3 is xe2x80x9cnormally transmissive.xe2x80x9d
When a xe2x80x9chighxe2x80x9d voltage is applied, shown in FIG. 3 to be 6V or higher, the liquid crystal aligns substantially normal to electrodes 12a, 16, or, equivalently, substantially parallel to the electric field between the electrodes. Such alignment of the liquid crystal corresponds to a xe2x80x9cdark statexe2x80x9d of the liquid crystal, where little or no light is transmitted.
Referring to FIG. 4, the alignment of the liquid crystal is shown for adjacent electrodes both having a xe2x80x9chighxe2x80x9d voltage magnitude, but where one voltage is positive and one is negative. Adjacent electrodes in this state are referred to as bring in an xe2x80x9cinversion mode.xe2x80x9d
Such application of opposing voltage is routinely practiced in the LCD arts and is for the purpose of reducing artifacts such as flicker and improving the overall uniformity of the display.
The liquid crystal generally designated as being in the xe2x80x9ccentralxe2x80x9d regions of both electrodes 12a, 12b in FIG. 4 are similarly aligned to give a dark state, like the alignment corresponding to 6V as shown in FIG. 3. The normal tilt inclination of the liquid crystal is the same in the central regions above both electrodes 12a and 12b, even though the potentials of the electrodes are 6V and xe2x88x926V, respectively.
The region spanning the gap between the electrodes 12a, 12b is generally designated as the xe2x80x9cinterpixel regionxe2x80x9d in FIG. 4. Moving from the central region of electrode 12a through the interpixel region and into the central region of electrode 12b, the electric field transitions from +6V in a direction perpendicular to electrodes 12a, 16 to xe2x88x926V in a direction perpendicular to electrodes 12b, 16. As shown in FIG. 4, above the interpixel region between the pixels 12a, 12b this electric field (12V) dominates the alignment of the liquid cystal, forcing it to align parallel to the substrate 10 surface in the interpixel region. (This strong parallel electric field also removes the normal helix-like alignment of the liquid crystal. That and the lack of a reflective surface between pixels 12a, 12b leads to little or no transmission of light in the interpixel region, as shown in FIG. 4.)
As shown, in the interpixel region the liquid crystal tends to align with this relatively strong electric field (approximately 12V, resulting from the composite electric field from electrodes 12a, 12b). As a result, at the right side of the gap region (i.e., above electrode 12b) the liquid crystal tends to tilt opposite its normal tilt inclination. This corresponds to the beginning of the transition of the strong electric field parallel to the electrodes in the interpixel region to a field of xe2x88x926V perpendicular to electrodes 12b, 16 at the central region of electrode 12b. 
Thus, moving from the interpixel region toward the central region of electrode 12b, the electric field decreases in magnitude and changes direction, from parallel with respect to the substrate 10 surface to perpendicular with respect to the substrate 10 surface. At a certain point, the influence of the elastic energy of the liquid crystal to align according to its normal tilt inclination exceeds the influence of the electric field to hold it opposite its normal tilt inclination. At that distance, the liquid crystal will transition from its opposite tilt to its normal tilt. As shown in FIG. 4, separating these two regions there exists an artifact of liquid crystal alignment. It is commonly referred to as a xe2x80x9cdisclinationxe2x80x9d by those skilled in the art, or, more specifically, as the xe2x80x9creverse tilt disclination.xe2x80x9d
This disclination is referred to as the reverse tilt disclination because it separate regions of opposing tilt. This disclination results in an unwanted transmission of light. The transmission of light results in a spurious bright line across a portion of an otherwise darkened pixel.
The voltage applied to the electrode will determine where the disclination forms on the pixel surface. When the disclination is sufficiently close to the edge of the pixel, it could be masked by a dark matrix preformed on the passive plate. However, alignment of the dark matrix is difficult, especially for high fill LCDs. Misalignment of the mask will result in a loss of light transmission, thus reducing efficiency. Even if the matrix is positioned correctly, it will block transmission of some light from the pixel when it is in a xe2x80x9celitxe2x80x9d state, thus reducing its efficiency. In such displays, avoiding use of a dark matrix mask is preferred.
It is thus an objective of the invention to reduce or eliminate the reverse tilt disclination on an LCD. It is also an objective to do so without a dark matrix mask, where there can be a significant loss of light transmission when the pixels of an LCD are in a lit state, and/or a loss of yield that can also result from misalignments of the dark mask (a loss that might otherwise be tolerable in low fill LCDs).
The present invention overcomes these disadvantages by eliminating the reverse tilt disclination. Because the disclination is eliminated through the internal make-up of the LCD, there is no need for a dark matrix mask and its resulting disadvantages.
In accordance with the present invention, the disclination is eliminated by displacing the bend deformation of the interpixel region. Displacement of the bend deformation is achieved by introducing a spacer material in the liquid crystal corresponding to the region between pixels. The spacer material displaces the liquid crystal, thus preventing development of the bend deformation and the resulting disclination when two adjacent pixels are operated in the inversion mode.